Schweitzer Engineering Laboratories SEL-787, Instruction Manual, Page: 417 / 564

E.3
Date Code 20081022 Instruction Manual SEL-787 Relay
Modbus RTU Communications
Communications Protocol
Modbus Exception
Responses
The SEL-787 sends an exception code under the conditions described in
Table E.3 .
In the event that any of the errors listed in Table E.3 occur, the relay assembles
a response message that includes the exception code in the data field. The
relay sets the most significant bit in the function code field to indicate to the
master that the data field contains an error code, instead of the required data.
Cyclical
Redundancy Check
The SEL-787 calculates a 2-byte CRC value through use of the device
address, function code, and data region. It appends this value to the end of
every Modbus response. When the master device receives the response, it
recalculates the CRC. If the calculated CRC matches the CRC sent by the
SEL-787, the master device uses the data received. If there is no match, the
check fails and the message is ignored. The devices use a similar process
when the master sends queries.
01h Read Discrete
Output Coil Status
Command
Use function code 01h to read the On/Off status of the selected bits (coils) (see
the Map shown in
Table E.14 ). You can read the status of as many as 2000 bits
per query, using the fields shown in Table E.4 . Note that the SEL-787 coil
addresses start at 0 (e.g., Coil 1 is located at address zero). The coil status is
packed one coil per bit of the data field. The Least Significant Bit (LSB) of the
first data byte contains the starting coil address in the query. The other coils
follow towards the high order end of this byte and from low order to high
order in subsequent bytes.
Table E.3 SEL-787 Modbus Exception Codes
Exception
Code
Error Type Description
1 Illegal Function
Code The received function code is either undefined or
unsupported.
2 Illegal Data
Address The received command contains an
unsupported address in the data field.
3 Illegal Data Value The received command contains a value that is out of
range.
4 Device Error The SEL-787 is in the wrong state for the function a
query specifies.
This also stands for Service Failure for DeviceNet
interface applications. The relay is unable to perform
the action specified by a query (i.e., cannot write to a
read-only register).
6 Busy The device is unable to process the command at this
time, due to a busy resource.
Table E.4 01h Read Discrete Output Coil Status Command (Sheet 1 of 2)
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (01h)
2 bytes Address of the first bit
2 bytes Number of bits to read
2 bytes CRC-16